1. Field of the Invention
This disclosure relates in general to tunnel magnetoresistive devices, and more particularly to a method and apparatus for oxidizing conductive redeposition in TMR sensors.
2. Description of Related Art
Magnetic recording media have been predominantly magnetic disks and magnetic tapes. They are manufactured by forming a thin magnetic film on an Al substrate or a resin tape. A magnetic head utilizing an electromagnetic conversion operation is used in order to write and read magnetic information to and from these magnetic media. This magnetic head comprises a write portion for writing the magnetic information to the recording medium and a read portion for reading out the magnetic information from the recording medium. A so-called “induction type head”, which comprises a coil and magnetic poles that wrap the coil from above and below and are electrically connected to the coil, is generally used for the write portion.
Magneto-resistive (MR) sensors based on anisotropic magneto-resistance (AMR) or a spin-valve (SV) effect are widely known and extensively used as read transducers to read magnetic recording media. Such MR sensors can probe the magnetic stray field coming out of transitions recorded on a recording medium by generating resistance changes in a reading portion formed of magnetic materials. AMR sensors have a low resistance change ratio or magneto-resistive ratio ΔR/R, whereas SV sensors have a much higher ΔR/R for the same magnetic field excursion. SV heads showing such high sensitivity are able to achieve very high recording densities.
In a basic SV sensor, two ferromagnetic layers are separated by a non-magnetic layer. An exchange or pinning layer is further provided adjacent to one of the ferromagnetic layers. The exchange layer and the adjacent ferromagnetic layer are exchange-coupled so that the magnetization of the ferromagnetic layer is strongly pinned or fixed in one direction. The magnetization of the other ferromagnetic layer is free to rotate in response to a small external magnetic field. When the magnetizations of the ferromagnetic layers are changed from a parallel to an anti-parallel configuration, the sensor resistance increases yielding a relatively high MR ratio.
Recently, new MR sensors using tunneling magneto-resistance (TMR) have shown great promise for their application to ultra-high density recordings. These sensors, which are known as magnetic tunnel junction (MTJ) sensors or magneto-resistive tunnel junctions (MRTJ), came to the fore when large TMR was first observed at room temperature. Like SV sensors, MTJ sensors basically include two ferromagnetic layers separated by a non-magnetic layer. One of the magnetic layers has its magnetic moment fixed along one direction, i.e., the fixed or pinned layer, while the other layer, i.e., free or sensing layer, is free to rotate in an external magnetic field. However, unlike SV sensors, this non-magnetic layer between the two ferromagnetic layers in MTJ sensors is a thin insulating barrier or tunnel barrier layer. The insulating layer is thin enough so that electrons can tunnel through the insulating layer. Further, unlike SV sensors, MTJ sensors operate in CPP (Current Perpendicular to the Plane) geometry, which means its sensing current flows in a thickness direction of a laminate film or orthogonal to the surfaces of the ferromagnetic layers.
The relative magnetic direction orientation or angle of the two magnetic layers is affected by an external magnetic field such as the transitions in a magnetic recording medium. This affects the MTJ resistance and thus the voltage of the sensing current or output voltage. By detecting the change in resistance and thus voltage based on the change in relative magnetization angle, changes in an external magnetic field are detected. In this manner, MTJ sensors are able to read magnetic recording media.
In the patterning of tunnel magnetoresistive (TMR) sensors, ion milling is commonly used in recording head structures and reactive ion etching (RIE) is commonly used in magnetic random access memory (MRAM) structures. While both of these techniques are effective in patterning the sensor material, both leave behind two artifacts, which create a parasitic resistance path, which is parallel to the remaining structure.
The first artifact is redeposited metal (redep). When patterning TMR structures it is quite common for some metal to be milled away from the field and redeposited on the sides of the TMR stack. However, this material can be conductive. The parasitic resistance created by this conductive material lowers the SNR in a functioning device.
The second artifact is ion damage to the barrier layer. TMR structures typically employ an oxide insulator barrier layer. The ion milling and RIE are both prone to damage the edge of the oxide insulator barrier layer and deplete it of oxygen. This also creates a parasitic resistance, which lowers SNR in a functioning device.
Present solutions include reactive ion oxidation of the redeposition metal. In reactive ion oxidation energetic oxygen ions are used to partially oxidize the redep and re-oxidize or “heal” the barrier layer. Nevertheless, reactive ion oxidation has two detremental effects. First, oxygen ions have the effect of reducing the thickness and dimensions of photoresists and carbon layers, which may be used to pattern the TMR device and its subsequent biasing layers. Secondly, oxygen ions can penetrate deeply into the sides of a TMR stack, reducing its effective area and damaging the free layer or pinned layer of such devices.
It can be seen that there is a need for a method and apparatus for oxidizing conductive redeposition in TMR sensors.